1. Field of the Invention
This invention pertains to the generation of x-rays and particularly to a method and apparatus for obtaining pulses of ionizing photons (X-rays and ultraviolet) resulting from the impact of free electrons emitted by a pulsed plasma on an anode in an impressed electric field.
2. Description of the Related Art
Widespread technological use has been made of the photocathode principle, according to which a negatively charged material is illuminated by light and the emitted electrons are then collected at an anode. Electrons emitted by a laser-irradiated photocathode have been accelerated in an electrostatic field, up to energies (18 kV) sufficient to perform 180 ps electron diffraction experiments on very thin metallic foils. In related work, the photoelectrons stimulated by an 18 ps laser pulse incident on a metallic cathode were accelerated in a static electric field, and used to produce an electron impact x-ray source of 70 ps duration. The electron current was space charge limited, even at 60 kV bias potentials.
Photoemission has been recognized for several years as a unique electron source. The energy distribution, the polarization, and the time profile of the electron beam can be carefully controlled by manipulating the wavelength, the polarization, and the time dependence of the excitation light source. Photoemission is induced by the linear photoelectric effect using picosecond pulses of a frequency-quadrupled Nd:YAG laser. For a full discussion of this technique, See, Van Wonterghem and Rentzepis, Characteristics of a Ta Photocathode for the Generation of Picosecond X-ray Pulses, Appl. Phys. Lett. 58(11), 12 Mar. 1990.